The inoculating loop is a vital piece of equipment in the quantitation of urine and other cultures. The loop must deliver a precise volume of liquid and accordingly must be subject to rigorous quality control to ensure that the volume of delivery is accurate. Any deformity or irregularity in the loop may affect its volume capacity.
Many manufacturers and larger laboratories typically use the Evans Blue Dye method to validate the calibration of metal loops. Significant time is required, however, to prepare the required dye solution, to measure the optical density of several known volumes of dye using pipettes and a spectrophotometer, to draw a standard curve reflecting the optical density of the known volumes, and then to compare the optical density of the amount of dye held in the loop to the curve. Furthermore, this method has a .+-.20% error.
In addition to the Evans Blue Dye method, Evergreen Scientific includes a gravimetric method using random urine samples in its quality control protocol. This procedure compares the tare weight and gross weight of the loop using an analytical balance. The difference between these two measures is the weight of the urine picked up by the loop. Precise physical measurements of the loop's inside diameter and thickness are also taken.
Many laboratory technicians, however, generally do not have the time or the inclination to validate loop calibration using either of these methods, yet they still prefer, and in some clinical studies such as urine cultures even require, some confirmation of loop accuracy.
One recent method approved by the FDA has provided technicians with a fast, simple way to validate calibration using two drill bits, each of slightly different size. These drill bits have known, fixed diameters that are resistant to damage. A loop of the proper diameter, and theoretically of the proper volume as well, will fit over a smaller bit but not over a larger one. Thus, by using drill bits of very slightly different sizes, it is possible to establish a narrow range of tolerance for the diameter of a loop. For example, a properly calibrated 1.mu.1 loop (a standard size) has an inside diameter of 1.45.+-.0.06 mm (0.057.+-.0.002 inches) and will fit over the end of a #54 drill bit but not over a #53 drill bit.
The drill bit method, however, does not measure the thickness of the loop, a dimension which also affects the volume held and delivered by the loop. Indeed, when using the gravimetric method, Evergreen found that increasing the thickness of the loop was highly correlated with increments of the loop's volume.